Shaun Evans is looking a bit battered and bruised. With cuts and scratches on his face, he looks as though he’s been in a lovers’ tiff or a pub brawl. And given that Shaun is playing the young Endeavour Morse, who spent many hours propping up the bars of Oxford, the latter is most likely.

Luckily, the abrasions are just make-up, and 34-year-old Shaun is otherwise in fine form. He’s back for a second, four-part series of Endeavour, the prequel to ITV’s much-loved Inspector Morse detective series, set in 1960s Oxford.

We meet on the set near Maidenhead in Berkshire, where the police HQ and Morse’s flat have been created in period detail. For Shaun, it’s a new experience to return to a role, and he’s keen that neither he nor the series appear to be coasting on their success.

“My feeling is, if you’re coming back to do it again, you need to up your game,” he says. “Those things we know about the character are in our subconscious, but this time you have to do the things you felt you didn’t achieve last time a little better.”

It’s an admirable sentiment from the actor, who’s had roles in Whitechapel, The Last Weekend and Silk, among others. Tackling Colin Dexter’s fictional hero has given Shaun a platform from which to flex his acting muscles, portraying a younger version of the melancholic, hard-drinking, irascible Morse that audiences grew to love when the late John Thaw played him from 1987 to 2000.

The previous series of Endeavour saw the young detective’s father die and Morse shot and injured. Now, it’s four months on and Morse has been moved to a different station, removing him from the kindly influence of DI Fred Thursday (Roger Allam).

“When we do come back this time, Morse is very much isolated and unhappy and not firing on all cylinders. A lot of that is to do with him drinking too much,” says Shaun. “There’s mileage in a story like this in that you get the opportunity to see Morse at home, so you understand why he’s behaving in a way that makes the rest of the office go, ‘What’s wrong with you, man?’

“Morse has inherited some of his father’s bills, so there’s financial pressure, and he has to look after his stepmum and sister. He’s in a slightly darker place, I think.”

While Shaun brings youthful energy to his role, Roger Allam, 60, brings a more measured wisdom as Morse’s mentor, Thursday. Roger, also known from BBC2’s political satire, The Thick Of It, says Thursday is concerned about Morse’s mental state.

“He’s concerned that Morse is drinking a bit too much and that it’s a sign that not all is well,” he says.

Allam appeared in a 1996 episode of Morse and is an acclaimed theatre actor who has largely stayed away from TV to avoid typecasting. “One of the things that attracted me to Fred Thursday is that he’s a character I’ve not played much of,” he says. “He’s down to earth, a good, pipe-smoking individual. I’d love to be in a western, and probably playing Thursday is the closest I’ll get to it, with the occasional bit of gunplay and fisticuffs.”

Roger, who grew up in London’s East End, has a personal reason for wanting to play Thursday, who fought in the Second World War before becoming a policeman.

“My father was a vicar in the East End just after the war,” he says. “My grandparents’ generation fought and lived through the First World War and my parents through the second, so war was still very present as I was growing up. One of the reasons I love playing Fred Thursday is that it’s a slight way of honouring that past and narrative and what people lived through.”

The new series sees Morse tackling murders against the background of the 1966 World Cup and Bonfire Night, hunting a fictitious Oxford Strangler and investigating a boys’ borstal and a private girls’ school.

With last year’s series of Endeavour achieving impressive viewing figures, Morse and Thursday have clearly struck a chord.